Add support for user defined "pool" to GN.

tools/gn/item.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/item.h"
 
 #include "base/logging.h"
 #include "tools/gn/settings.h"
 
 Item::Item(const Settings* settings, const Label& label)
     : settings_(settings), label_(label), defined_from_(nullptr) {
 }
 
 Item::~Item() {
 }
 
 Config* Item::AsConfig() {
   return nullptr;
 }
 const Config* Item::AsConfig() const {
   return nullptr;
 }
+Pool* Item::AsPool() {
+  return nullptr;
+}
+const Pool* Item::AsPool() const {
+  return nullptr;
+}
 Target* Item::AsTarget() {
   return nullptr;
 }
 const Target* Item::AsTarget() const {
   return nullptr;
 }
 Toolchain* Item::AsToolchain() {
   return nullptr;
 }
 const Toolchain* Item::AsToolchain() const {
   return nullptr;
 }
 
 std::string Item::GetItemTypeName() const {
   if (AsConfig())
     return "config";
   if (AsTarget())
     return "target";
   if (AsToolchain())
     return "toolchain";
+  if (AsPool())
+    return "pool";
   NOTREACHED();
   return "this thing that I have no idea what it is";
 }
 
 bool Item::OnResolved(Err* err) {
   return true;
 }